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I have a sensor that measures temperature, relative humidity and air pressure.

All the formulas I could find for both absolute humidity and the dew-point only use the temperature and humidity values, never the pressure.

I wonder if that is because:

A) The relatively small pressure differences from 1atm that occur naturally (outside of artificial pressure vessels) have effects that are too small to be relevant for the realistically achievable measurement accuracy in usual settings. Or:

B) The relative humidity changes proportional to the air pressure in relation to the formulas for absolute humidity and dew point, so that it's effect is already considered through the relative humidity value. Or:

C) Nobody is as pedantic of getting the highest accuracy possible as me ;)

I consider A or B to be the more likely explanations, but I'd love to hear the input of more knowledgeable people on the point.

This question seems related: What law or formula discusses the relationship between pressure and dew point?

Reading this confirmed my suspicion that changing the pressure (&volume) of a given gas mixture containing water vapor will change it's dew point.

But I'm unsure if remeasuring temperature (which I believe would be raised by increased pressure) and relative humidity (unsure if and how that would change) would result in values that when used to calculate a fresh dew-point would accurately reflect the change caused by the new air pressure.

UPDATE: So I found this calculator: https://airpack.nl/tools/dew-point-calculator/ Not as informative as a formula would be, but playing around with it showed me that a change of just 50 mbar to a air-mixture with dewpoint 15°C will change the dewpoint by ~1°C - so option A) seems be much less likely to me now.

UPDATE2: I thought I should add the formula I'm currently using to calculate the dew-point:

let a = (17.67 * self.temperature_c) / (243.5 + self.temperature_c);
let b = (self.rel_humidity_pct / 100.0).ln();
(243.5 * (b + a)) / (17.67 - b - a)
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  • $\begingroup$ This is what steam tables are for, pretty much... $\endgroup$
    – Jon Custer
    Aug 28, 2023 at 19:43
  • $\begingroup$ Thanks for your comment @JonCuster - Can you expand on this please? What are steam tables, and what are they for? $\endgroup$
    – xogoxec344
    Aug 29, 2023 at 8:40
  • $\begingroup$ Well, steam locomotives are not so common anymore... But, steam tables list the properties of steam as a function of temperature, including the saturated vapor pressure. See, for example, thermopedia.com/content/1150 $\endgroup$
    – Jon Custer
    Aug 29, 2023 at 12:10
  • $\begingroup$ Maybe I misunderstood but it seems that would be applicable for calculating properties of steam, i.e. 100% water, not a regular air mixture, no? $\endgroup$
    – xogoxec344
    Aug 29, 2023 at 19:29
  • $\begingroup$ Does this answer your question? What law or formula discusses the relationship between pressure and dew point? $\endgroup$ Aug 29, 2023 at 19:49

1 Answer 1

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I believe I have found an answer, I'm still not 100% sure but here it is:

https://en.wikipedia.org/wiki/Vapor_pressure#Meaning_in_meteorology

... Actually, as stated by Dalton's law (known since 1802), the partial pressure of water vapor or any substance does not depend on air at all, ...

Since the dew-point by definition is the temperature at which the current partial pressure of water vapor is equal to the saturation vapor pressure, I believe I can infer from above statement that the dew point also does not depend on any properties of the air except the partial pressure and temperature of the water vapor contained in it.

I'd love to read comments confirming or denying either my conclusion, or the quote from Wikipedia I'm basing it on.

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